A grid-tied electrical system, also known as a tied-to-grid system or a grid-tie system, is a system that generates electricity and provides the electricity to an electrical grid to which the system is tied. Traditionally, the grid-tied electrical system utilizes renewable energy sources such as the sun or wind. For example, the grid-tied electrical system may convert solar energy into electricity via photovoltaic effects.
FIG. 1 illustrates a block diagram of a conventional grid-tied electrical system 100 to provide a single-phase alternating current (AC) to an electrical grid 101 outputting an AC voltage, referred to herein as a grid voltage Vgrid. Referring to FIG. 1, the system 100 is tied to the electrical grid 101, and includes a photovoltaic (PV) array 102, a maximum power point tracking (MPPT) module 104, a first capacitor 106, a first switch 108, and a transformer 110. The system 100 further includes a rectifier 112, a second capacitor 114, and a plurality of switches including, e.g., switches 116-1, 116-2, 116-3, and 116-4.
More particularly, the PV array 102 converts solar energy into direct current (DC) electricity via photovoltaic effects. The MPPT module 104 is coupled to the PV array 102, and is configured to track a maximum power point (MPP) of the PV array 102 and to provide to the transformer 110 an MPP voltage at a relatively low voltage level. A primary side of the transformer 110 is coupled to the MPPT module 104 through the switch 108, and is configured to convert the MPP voltage at the relatively low voltage level to a converted voltage VT0 at a relatively high voltage level based on a transformer turns ratio, when the switch 108 opens and closes under control of a predetermined control signal Sa′. When the switch 108 opens, the primary side of the transformer 110 is also open. When the switch 108 closes, the primary side of the transformer 110 is connected to the capacitor 106 though ground. Through this open/close mechanism, the switch 108 performs pulse width modulation (PWM) and transfers energy from the primary side of the transformer 110 to the secondary side of the transformer 110.
The rectifier 112 is coupled to a secondary side of the transformer 110, and is configured to convert the voltage VT0, which is generally an AC voltage, to a DC voltage Vdc. The DC voltage Vdc is further smoothed by the capacitor 114. The switches 116-1, 116-2, 116-3, and 116-4 are operable to generate an AC voltage equal to the grid voltage Vgrid, when the switches 116-1 and 116-2 alternately close under control of predetermined control signals S1′ and S2′, respectively, and the switches 116-3 and 116-4 alternately close under control of predetermined control signals S3′ and S4′, respectively.
FIG. 2 shows a simulation result including waveforms of the predetermined control signals Sa′, S2′, S3′, and S4′ applied to the conventional system 100 (FIG. 1) and the voltages VT0, Vdc, and Vgrid described above. To show more detail, the voltages VT0, Vdc, and Vgrid, and the control signals Sa′, St, S2′, S3′, and S4′ during time periods t1, t2, and t3 have been enlarged. Each of the predetermined control signals Sa′, St, S2′, S3′, and S4′ is a periodic pulse signal that has a relatively high frequency. The control signals Sa′, St, S2′, S3′, and S4′ switch on/off the switches 108, 116-1, 116-2, 116-3, and 116-4, respectively, with different timings. Further, as shown in FIG. 2, the voltage VT0 is an AC voltage having a relatively high frequency and a relatively high voltage level, and the voltage Vdc is a DC voltage.
Referring to FIGS. 1 and 2, the switches 116-1, 116-2, 116-3, and 116-4 operate at the relatively high frequency under the control of the predetermined control signals St, S2′, S3′, and S4′, respectively. In addition, the capacitor 114 operates at the relatively high voltage level. As a result, a life time of the system 100 may be reduced due to high system wear.